This application relates to polyurethane compositions useful as adhesives, coatings, sealants and elastomers, especially cast polyurethane elastomers.
High service temperatures are required for many applications for polyurethane compositions. High service temperature is directly related to the thermal stability of the polyurethane, which is usually expressed in terms of the specific temperature, temperature ranges, or time-temperature limits, within which the polymer can be used without it experiencing significant degradation of key performance properties. It is known that conventional polyether based polyurethane compositions can withstand continuous use temperatures of about 100 to about 120.degree. C. The use of polyurethane compositions in automotive applications, such as under the hood, and electrical encapsulation or bonding applications often require higher service temperatures of about 140 to about 155.degree. C. As polyether based polyurethane compositions provide processing cost advantages over other polyurethanes it is desirable to have high service temperature polyether based polyurethane compositions. Such high service temperature applications include elastomers used in electrical castings, adhesives and coatings, encapsulants and gaskets.
Cast elastomers may be used for a wide variety of applications including tires and wheels, for example forklift tires, roller-skate and roller-blade wheels, running shoes, brake diaphragms, snowplow blades, grain buckets, drilling pipe thread protectors, grain and coal shoots, classifier shoes, hydraulic seals, wheel shocks, bowling ball cover stock, shaft couplers, sheet goods, rod stock, mining screens, conveyor belts, coated conveyor belts, gears, pipeline rigs, boat fenders, bump liners, helicopter blade sleeves, bumper pads, die cut pads (paper box industry), large rollers for steel and paper mills, copier rolls, encapsulated gate valves, encapsulated transponders (cattle tags), encapsulated concrete mixer blades, drive belts, dead blow hammers, sprockets, overrings, crane shock absorbers, sound dampening pads potting compounds and for coatings and encapsulated parts which are used in high heat environments, such as in the engine compartment of automobiles. Polyurethane cast elastomers are typically made by contacting the raw materials such as a polyisocyanate, a polyol, and often a chain extender in a mold coated with a mold release, and the contacted material is allowed to cure to form an elastomeric polyurethane piece.
Polyurethane rigid foam utilizing highly functional rigid polyols, have been disclosed as useful in the insulation of hot water/steam pipes for district heating, see Pots, et al., Developments In Rigid Polyurethane Foams for Insulation of District Heating Pipes, Polyurethanes World Congress, 1984. It is recognized that higher heat resistance is not normally obtained using conventional polyurethane chemistry except through the modification of the chemical structure by adding heterocyclic groups such as isocyanate, poly(urethane-oxylazolidone-isocyanate),or polyimide groups. See Frisch, et al., New Heat Resistant Isocyanate Based Foams For Structural Applications, Polyurethanes World Congress, 1991 and Frisch, et al., Novel Heat Resistant Isocyanate Based Polymers, 33.sup.rd Annual Polyurethane Technical Marketing Conference, Sep. 30-Oct. 3.sup.rd 1990. There are some limitations with this chemistry and soft elastomer products having such properties are difficult to obtain. The open literature has reported the development of heat resistance elastomers having been achieved through the use of specific cross-linkers, such as p,p' diphenol, quinol and hydroquinone di(beta-hydroxyl ethyl) ether (HQEE) and the like, see Frisch supra. It has also been reported that high heat resistant elastomers can be prepared from paraphenylene diisocyanate and naphthalene diisocyanate, Hepburn Polyurethane Elastomer 2.sup.nd Edition, 3:67, 1964 and Plummer, et al. Paraphenylene Diisocyanate Based Thermoplastic Polyurethane Provide High Thermal Stability, Polyurethanes Expo 1996. The high temperature elastomer systems described in such references exhibit the drawback of being solid or containing compounds solid at room temperature and therefore require processing above their melting points, usually above 100.degree. C.
Conventional polyether based polyurethane elastomers can withstand continuous use temperatures of up to 100.degree. C. to 120.degree. C. European community legislative changes relating to vehicle pass by noise have reduced the allowed noise emission. This requires engine encapsulation and reduced air flow within the engine compartment resulting in higher working temperatures for all materials used in the engine compartment. Elastomers, gaskets and electrical encapsulating applications now require higher working temperatures, up to about 140.degree. C. with a peak application temperature of up to about 155.degree. C. Until now no heat resistance soft cast elastomers based on conventional polyether based polyurethane systems have been available.
What are needed are polyurethane formulations capable of forming polyurethanes which can be used in high temperature service environments and which do not degrade when exposed to temperatures of 120.degree. C. or greater, preferably about 140.degree. C. to about 155.degree. C. and which formulations are easy to process at about ambient temperatures. In other words, formulations which are low in viscosity and easy to handle at low temperatures. What are also needed are high temperature resistant elastomers which can be prepared by casting techniques at or near ambient temperatures.